Luminance correction for color scanning using a measured and derived luminance value

ABSTRACT

A color scanning technique that maintains a high image quality while permitting improved scanning speed and improved perceived resolution. A color pixel array and a luminance pixel array are generated representative of a target object. The pixel arrays are related in that one or more luminance pixels of the luminance pixel array cover each pixel of the color pixel array. A luminance value is sensed for each luminance pixel, and three primary color values are sensed for each color pixel. A measured luminance value is then associated with each respective color pixel wherein the measured luminance value is a function of the sensed luminance values for the one or more luminance pixels covering the respective color pixel. A derived luminance value is also calculated for each respective color pixel wherein the derived luminance value is a function of the three sensed primary color values for the respective color pixel. A luminance correction factor is then determined for each respective color pixel as a function of the color pixel&#39;s derived and measured luminance values. The luminance correction factor for each respective color pixel is applied to the sensed primary color values of the color pixel, or to a linear combination thereof, to determine luminance-corrected color values. In this way the overall pixel luminance implied by the aggregate measured color values will show a consistency with the directly measured luminances. The luminance-corrected color values together with the measured luminance values may then be subjected to further appropriate transformation to determine color coordinates for each luminance pixel and/or for each color pixel, which may then be used in further processing. A logarithmic method is used for efficient computation of correction factors and luminance-corrected color values.

BACKGROUND OF THE INVENTION

The present invention relates to scanning of color images and is moreparticularly directed to techniques for maintaining image quality andespecially for maintaining image quality while scanning at higherspeeds.

Optical image scanners convert an object to be scanned such as a printeddocument, photograph, transparency or other image or scene into adigital electronic signal representative of the scanned object. Theelectronic signal may then be subjected to further processing andanalysis and sent to an output device such as a printer or displaymonitor.

The image is typically captured by a sensor responsive to light from thetarget. For gray scale images (that is, so-called “black and white”images) a sensor is used that is responsive only to the luminance of thelight from the scanned object and that does not distinguish colors. Forcolor images a sensor is used that is separately responsive to the red,blue and green primary color components of the light from the target.

The sensors partition the image into arrays of pixels and associate aluminance value or red, blue and green values with each pixel.Commercially available gray scale sensors are generally capable ofperforming at higher speed and higher resolution than commerciallyavailable color sensors. Gray scan sensors typically operate in therange of 10 to 100 megapixels per second (MPix/s) while color sensorsoperate at 1 to 20 MPix/s. In comparing color and gray-scale rates, onecolor pixel is considered to have three color data channels associatedwith it for the three primary colors. Color scanners perform more slowlyin part because of the increased amount of data they must scan andprocess.

In an attempt to improve the scanning speed of color images and improveimage quality, some scanners include both luminance sensors and colorsensors and scan the luminance data and the color data at two differentresolutions. See for example U.S. Pat. Nos. 5,045,932 and 5,619,590. Ahigh-resolution luminance sensor is used to capture the image detail,and a lower-resolution color sensor captures the color information. Thedata from the two sensors are then combined according to an appropriatescheme to provide an output signal. While such schemes generally show animprovement in scanning speed, and are sometimes able to maintain goodimage quality, they nevertheless represent a compromise in the qualityof the original image.

SUMMARY OF THE INVENTION

The present invention provides a color scanning technique that maintainsa high image quality while permitting improved scanning speed andimproved perceived resolution. It is an object of the invention toachieve high-resolution color scans and particularly to improve upon theresolution that has generally been realized from commonly availablecolor charge-coupled device (CCD) sensors. It is another object of theinvention to achieve higher effective scanning speeds than has generallybeen realized with the commonly available color CCD sensors. It is yetanother object of the invention to provide improved color coordinates,that is to say, values associated with each pixel, encoding the colorinformation for each pixel. An important aspect of the invention is thatthe luminance data channel determines the overall luminance of thecombined output signal, while the color data channels supply only thecolor information.

Briefly, a target object is scanned in accordance with the invention bygenerating a color pixel array representative of the target object, orat least of the portion of the object of interest, and also generating aluminance pixel array representative of the same portion. The colorpixel array and luminance pixel array will generally be different, butthey are related in that one or more luminance pixels of the luminancepixel array cover each pixel of the color pixel array. A luminance valueis sensed for each luminance pixel, and three primary color values aresensed for each color pixel. A measured luminance value is thenassociated with each respective color pixel wherein the measuredluminance value is a function of the sensed luminance values for the oneor more luminance pixels covering the respective color pixel. Inaddition, a derived luminance value is also calculated for eachrespective color pixel wherein the derived luminance value is a functionof the three sensed primary color values for the respective color pixel.A luminance correction factor is then determined for each respectivecolor pixel as a function of the color pixel's derived and measuredluminance values. The luminance correction factor for each respectivecolor pixel is applied to the sensed primary color values of the colorpixel, or to a linear combination thereof, to determineluminance-corrected color values. In this way the overall pixelluminance implied by the aggregate measured color values will show aconsistency with the directly measured luminances that has notheretofore generally been realized or appreciated in the scanning field.The luminance-corrected color values together with the measuredluminance values may then be subjected to further appropriatetransformation, if desired, to determine color coordinates for eachluminance pixel and/or for each color pixel. The color coordinates sodetermined provide an improved representation of the target object imageexhibiting improved image quality, which may then be used to advantagein further customary image processing functions such as imagecompression, filtering and analysis, storage, transmission, printing ordisplay.

In practice, determining and applying the luminance correction pixel bypixel can place a large computational burden on the system, which couldappreciably slow down the processing. This comes about because thedetermination and application of the luminance correction factor willgenerally require pixel-by-pixel multiplication and division operations,which are demanding on system resources. The computational burden isgreatly reduced, however, and the system processing rate is maintained,by converting the computationally laborious multiplication and divisionoperations to much simpler addition and subtraction operations performedon the logarithms of the appropriate quantities.

The invention may advantageously be practiced with luminance and colorsensors of high scanning rates so that the overall system will exhibit ahigh scanning rate. In so doing, the luminance correction of the presentinvention will provide a superior image quality to that achievable atthe same scanning rate without the benefit of the invention. Moreover, ahigh-resolution image with improved image quality may be achieved with ahigh-resolution luminance sensor generally of higher resolution than thecolor sensor. Even with luminance and color sensors of same resolution,i.e., luminance pixels and color pixels of the same size, an improvementin image quality still results because the color values are corrected tobe consistent with the true overall luminance value.

Other aspects, advantages, and novel features of the invention aredescribed below or will be readily apparent to those skilled in the artfrom the following specifications and drawings of illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a scanning apparatus.

FIG. 2 is a block diagram showing the processing of the color datastream.

FIG. 3 is a block diagram showing the processing of the luminance datastream.

FIG. 4 is a block diagram of a particular embodiment of a colorprocessing unit for practicing the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an idealized diagram of scanning apparatus for practicingthe invention. The scanning apparatus includes a light source 10 forilluminating the object to be scanned, which is taken here to be adocument 11 containing a color image. In a typical scanning apparatus,light source 10 generates a light beam that is directed to illuminate ahorizontal strip across the document generally referred to as ascanline. The light beam reflecting off the document carries a colorimage of the scanline. An optical system represented diagrammatically bylens 12 focuses the reflected scanline beam and directs it to a beamsplitter 13. One portion of the beam from the beam splitter is directedto a luminance sensor 14 and a second portion is directed to a colorsensor 15. The structure and operation of such scanning apparatus iswell known and is described herein only to the level of detail needed todescribe the present invention.

For realistic image reproduction it is desirable that the response ofluminance sensor 14 to the light reflected from the document correspondat least roughly to the response of a human observer to the document asilluminated by sunlight. To achieve this result, the signal from theluminance sensor should be properly color-balanced, for example, basedon the NTSC standards. As is known in the art, this can be done with ajudicious choice of phosphors in a fluorescent lamp or choice of filtersin front of a variety of light sources having a variety of spectralcharacteristics such as fluorescent lamps characterized by spectralspikes and broad-spectrum incandescent and halogen lamps. Regardless ofthe shape of the spectrum from light source 10, however, the luminancesensor is better able to sense the luminance content of the targetobject than the color sensor, which includes necessarily imperfectbandpass filters for sensing color content. The invention takesadvantage of this fact by correcting the color information for luminancecontent sensed by luminance sensor 14.

For the sake of illustration the luminance sensor may be provided by a4096-element linear CCD array such as the P-series linear arrayavailable from Reticon. The color sensor may be provided by a2048-element linear CCD array such as the model KLI-2113 array availablefrom Kodak. Although the invention is illustrated here with lineararrays and single-line scans, it may also be implemented with areaarrays, and those skilled in the art will be readily be able to adaptthe method for use with area arrays given the explanations and examplesprovided below.

The luminance and color sensors generate independent pixel arraysrepresentative of the target scanned object. For example, in the4096-element Reticon array the pixels are arranged on 7-micron centers,and in the 2048-element Kodak array they are on 14-micron centers. Thus,for these sensors a color pixel is twice as wide and twice as tall as aluminance pixel. This means that two linescans with the luminance sensorcorrespond to one linescan with the color sensor and that each pixelfrom the color array covers the same area as, and, ideally at least, maybe matched up with, a square composed of four luminance pixels (twoadjacent pixels from each of two adjacent linescans). The correspondenceof four luminance pixels to one color pixel is chosen for convenience ofillustration and for ease of alignment in an actual embodiment. However,other numbers and geometrical relations of the two kinds of pixels mayalso be used.

In general, the luminance pixels are defined in a pattern covering theindividual color pixels such that one or more luminance pixels forms atiling of each color pixel. In the embodiment described here, foursquare luminance pixels cover one square color pixel. Other patterns mayalso be utilized. While improved resolution can be achieved if severalluminance pixels correspond to one color pixel, some benefits of theinvention will nevertheless be realized even if the luminance and colorpixels are of the same size, that is, only one luminance pixelcorresponds to each color pixel. Moreover, while the luminance and colorpixels will preferably be aligned with one another, small discrepanciesin the alignment may be tolerated. The important point is that theluminance of the luminance pixels covering a color pixel be determinedsubstantially by the light coming from the area of the underlying colorpixel. Thus, small misalignments of the color pixels and theircorresponding luminance pixels can be tolerated to the extend that thecontribution to the sensed luminance values from any misaligned portionsis not significant.

The processing of the color data stream from color sensor 15 will now bedescribed with reference to FIG. 2. Color sensor 15 provides a datastream composed of the three primary color values, taken here as red(R), green (G) and blue (B), associated with successive color pixels insuccessive linescans. The data streams from color sensor 15 are sent tothree analog-to-digital (A/D) converters 16, which provide digital R, Gand B data streams.

As is well known in the operation of color CCD sensors, many colorsensors scan the individual pixels for their R, G and B values atdifferent times. Processing block 17 applies appropriate delays so thatall the color data from one pixel come out simultaneously. That is tosay, this block aligns the R, G and B components of a pixel so that theylie on top of one another. Offset and gain correction are also performedat block 7. These operations are known in the art and need not bedescribed in detail here. The output of block 17 is composed of threedata streams of R, G and B data. In the current embodiment each R, G andB data stream operates at a 10MHz rate. Multiplexer 18 receives thesedata streams and provides a multiplexed data stream operating at fourtimes the clock rate of block 17 (i.e., at 40-MHZ) and consisting of thecomponents GGRB, where the second green (G) component is repeated in thedata stream to serve as a placeholder for another value to besubstituted later.

Block 19 receives the GGRB data stream from multiplexer 18 andcalculates a luminance value for the individual pixels from their R, Gand B values. The calculated luminance value is denoted Y(RGB). This isa luminance value that is derived entirely from the data sensed by colorsensor 15 and does not derive from data sensed by luminance sensor 14.The specific calculation of the luminance value will be described inmore detail below. The calculation is preferably based on a generallyaccepted luminance standard. For the specific embodiment described belowthe luminance is based on the MPEG definition, although otherdefinitions could also be used consistent with the invention.

The value of Y(RGB) determined at block 19 is now substituted in placeof the second G component in the GGRB data stream at multiplexer 21. Theoutput of multiplexer 21 is then a multiplexed data stream withcomponents GYRB for each pixel sensed by color sensor 15.

The processing of the data from luminance sensor 14 is shown in FIG. 3.The data stream or streams from luminance sensor 14 is digitized by A/Dconverter(s) 23. The digital data streams are sent to processing block24 where various image processing steps may be performed. The datastream may be adjusted to compensate for gain and offsets. This servesto calibrate the data stream so that pure black will correspond to aluminance value of zero and full white to a value of unity. At thisstage the luminance data stream is aligned so that each successive 2×2block of luminance pixels sits on top of a corresponding color pixel.The data stream coming out of processing block 24 represents calibratedluminance pixels aligned on top of corresponding color pixels. Any othertechniques may also be used for aligning a set of luminance pixels withthe corresponding color pixel or pixels, and such techniques, while theymay represent different tradeoffs in performance characteristics andease of implementation, are considered equivalent for purposes of thepresent invention.

The data stream from processing block 24 comes out as a standard rasterstream of pixels at 40 Mpix/s. This data stream is stored and used asthe luminance component of the scanned image. The data stream is alsorearranged at multiplexer 26 as two neighboring line scanssimultaneously at 20 MPix/s. That is, a pair of linescans is presentedsimultaneously. The pair aligns with a single linescan from color sensor15, which has pixels that are twice as tall as a linescan from luminancesensor 14. The 20-MHZ linescan pairs are multiplexed into a single videostream at 40 MHz for convenience in further processing. At this stagethe data corresponding to a 2×2 block of luminance pixels in theluminance data stream from multiplexer 26 comes from the same place inthe scanned document as the corresponding pixel in the color data streamfrom multiplexer 21. At block 27 the luminance values for the fourluminance pixels aligned with a given color pixel are averaged toprovide a single luminance value representative of the color pixel. Thisvalue is denoted Y(Lum) to indicate that it is a function of the fourmeasured values stemming from the luminance sensor.

In the idealized situation the luminance value Y(Lum) of the spot on thedocument corresponding to any given color pixel as measured by luminancesensor 14 should agree with the luminance value Y(RGB) calculated fromthe R, G and B values measured by color sensor 15. In practice, however,these values do not always agree. A noticeable improvement in imagequality has been realized in the present invention by adjusting colorsensor measurements, and specifically by adjusting the R and B values,to match the higher-resolution measured luminance values whilepreserving the saturation and hue values measured by the color sensor.Moreover, this can be accomplished without any compromise in theprocessing speed.

For a greater understanding of the manner in which the luminanceadjustment to the color pixels is made it is beneficial to discussbriefly the theory of color image processing. In image systems theprimary colors R, G and B are defined on a relative scale from 0 to 1chosen such that shades of gray are produced when R=G=B. White is givenby R=G=B=1 and black by R=G=B32 0. The values of R, G and B allcontribute to the perceived brightness, but must be weighted accordingto the relative sensitivity of the eye to each primary color.

The eye is most sensitive to green light, less sensitive to red light,and least sensitive to blue light, and the eye's ability to resolvespatial detail is best in the greens at the center of the visualspectrum and poorest in the blue. Color image compression schemes takeadvantage of this circumstance since there is no reason to accuratelyreproduce details that the eye cannot see. According to the MPEGstandard the perceived luminance Y(RGB) as a function of the R, G and Bvalues is taken as

Y(RGB)=0.299*R+0.587*G+0.114*B.

The luminance Y(RGB) determined from the color channel at block 19 iscalculated according to this equation.

It is noted that MPEG encoding is based on YUV encoding, where Uapproximately encodes the blue-yellow hue/saturation and V approximatelyencodes the red-green hue/saturation. The U and V values are defined as:

U=B-Y(RGB)=0.886*B - (0,587*G+0.299*R),

V=R-Y(RGB)=0.701*R- (0,587*G+0.114*B).

The parameters U and V can take on positive and negative values that canbe inconvenient for processing. To eliminate negative values and giveall three components roughly the same dynamic range, U and V are scaledand zero-shifted through a linear transformation to the colorcoordinates Cb and Cr, which are used in MPEG compression schemes:

Cb=(U/2.0)+0.5=B/2.0−Y(RGB)/2.0+0.5,

Cr=(V/1.6)+0.5=R/1.6−Y(RGB)/1.6+0.5.

The luminance Y is the same for the YUV and YCbCr color coordinatesystems.

The use of the above equation for Y(RGB) in the present invention isconvenient because it is consistent with the MPEG compression standardand is commonly used for JPEG compression. Other definitions ofperceived luminance Y could also be used at block 19, however,consistent with other color coordinate or compression schemes.

The invention enforces a greater level of consistency in the processingof the color and luminance data. The higher-resolution luminance channelcontrols the luminance of the combined image data while the colorchannel controls the color. If no consistency between the channels wereenforced, values of R, G and B significantly different from the originalvalues, possibly even falling outside the permissible range couldoccasionally result if the inverse MPEG equations were applied to thedata from the separate channels, leading to color shifts and excess orwashed out color saturation.

The dominant factor in preserving image quality when forcing consistencybetween the luminance and the color channels is that the relativeamplitudes of the red, green, and blue values for each color pixel bekept the same. If the three color values for a pixel are multiplied ordivided by the same constant, only luminance of the pixel will beaffected and the hue and saturation will remain unchanged. Consistencymay therefore be enforced by applying an appropriate scaling factor,referred to as a luminance correction factor, to the color values fromthe color channel, which scaling factor may vary from color pixel tocolor pixel, so long as the same scaling factor is applied within apixel.

Good results are achieved if the scaling factor for a color pixel istaken to be

Y(Lum)/Y(RGB),

where Y(Lum) is the average of the luminance values from the fourluminance pixels making up the color pixel:

Y(Lum)=(sum of color pixel's four luminance channel values)/4

That is to say, the color values of each color pixel are scaled to thepixel luminance from the luminance channel. In particular,

R*Y/Y=R*Y(Lum)/Y(RGB),

B*Y/Y=B*Y(Lum)/Y(RGB).

This result is used if

Y(Lum)/Y(RGB)<ThresholdA,

and/or

Y(RGB)>ThresholdB,

where ThresholdA and ThresholdB are empirically selected parameters,which effectively determine whether there is enough light contrast towarrant using the color information. Color pixels falling below thesethresholds do not have sufficient luminance content to warrantcalculating a ratio. Otherwise,

R*Y/Y=B*Y/Y=Y(Lum)

is used, which avoids color fringing.

If the YUV or equivalently YCbCr color coordinates are used, there is noneed to adjust the G value, which does not appear in the calculation ofthese coordinates.

In terms of the adjusted R and B values the color coordinates Cb and Crbecome

Cb=(B*Y/Y)/2.0−Y(Lum)/2.0+0.5,

Cr=(R*Y/Y)/1.6−Y(Lum)/1.6+0.5.

While these color coordinates will achieve the advantage of improvedimage quality, they place a significant load on computational processingsince multiply and divide operations must be performed with differentmultipliers and divisors for each color pixel.

The computational complexity may be reduced to a more manageable loadwith the aid of log and anti-log table lookups, which convert themultiplications and divisions to additions and subtractions. First theY(Lum) data from the luminance data channel is multiplexed into the GYRBdata stream in the color data channel, substituting Y(Lum) for G. Thedata stream now consists of Y(Lum), Y(RGB), R and B, referred to as theYYRB data stream. This is passed to a Log Table which returns themantissa (the fractional portion of the log) and which in effect setsthe logs of very small numbers to large numbers within a practicalrange, while the integer portion of the log is calculated separately.The luminance correction factor LCF is then calculated as

Log_(—) LCF=Log_(—) Y(Lum) −Log_(—) Y(RGB).

and the following Log_YRB video stream is calculated and input to theanti-log lookup table, where Log_(—)1.6 is a constant:

Log_(—) Y/1.6=Log_(—) Y(Lum)−Log_(—)2.6,

Log_(—) R*Y/Y/1.6=Log_(—) R+Log_(—) LCF−Log_(—)1.6,

Log_(—) Y=Log_(—) Y(Lum),

The 8-bit YRB video stream out of the anti-log table therefore holds thevalues

Y/1.6=Y(Lum)/2.6,

R*Y/Y/1.6=R*Y(Lum)/Y(RGB)/1.6,

Y/1.0=Y(Lum), and

B*Y/Y=B*Y(Lum)/Y(RGB).

These values may then be used in the equations for Cb and Cr, which nowonly require simple additions and subtractions.

Having described the theory of the invention, a description is now givenof a particular embodiment for implementing the above calculations withreference to FIG. 4. Data lines 31 correspond to the line 31 from thealignment block 127 of FIG. 2. These data lines carry the aligned andotherwise pre-processed color values for one pixel. As described above,multiplexer 18 receives these data lines and provides a multiplexed datastream GGRB operating at four times the data rate of lines 31. LookupTable 32 and adder 33 implement the luminance calculator shown in block19. Together they implement the calculation

Y(RGB)=(0.299*1024)*R+(0.587*1024)*G+(0.114*1024)*B.

The result is reintroduced into the multiplexed data stream with the aidof delay 34 and multiplexer 35 to provide multiplexed GYRB data stream36.

The aligned and otherwise pre-processed raster luminance data line 40 isrearranged into 2×2 blocks or tiles using delays 41 and 42 andmultiplexer 43 to provide tiled luminance data along line 44. At thispoint the color and luminance data along lines 36 and 44 may besubjected to linear image processing such as resolution scaling shown atblocks 37 and 46. The output of image processor/scaler 46 isre-rasterized at block 48 and provides a full-resolution luminanceoutput on line 49. The output from image processor/scaler block 46 isalso passed through 2×2 averager 51 and then to multiplexer 52, where itis multiplexed with the data on line 38 to provide the YYRB data streamon line 53. The 10-bit multiplexed YYRB data stream is passed to lookuptable 54 and to mantissa calculator 55. The lookup table and mantissacalculator provide a 12-bit logarithm on line 56. At this stage a datastream is available on line 56 which is the log of the measuredluminance value (Log_YL), followed by the log of the derived luminancevalue (Log_YK), followed by the log of the measured Red value (Log_R),followed by the log of the measured Blue value (Log_B). Log _YK issubtracted from Log_YL at subtracting accummulator 57 and the result isprovided as an input to add/pass block 58. Block 57 also compares Log_YKand Log_YL−Log_YK with Log_ThresholdB and Log_ThresholdA, respectively,and supplies input correct/pass to multiplexer 61. Meanwhile, YL and YKare passed through block 58 to line 59. The output of block 57 is addedis at block 58 to Log_R and Log_B and provided along line 59. Register60 and multiplexer 61 rearrange the data to form a stream 62 in theformat Log_YL, Log_R*Y/Y (or Log_YL if correct/pass==pass), Log_YL,Log_B*Y/Y(or Log_YL if correct/pass==pass). Data stream 62 is passedthrough adder 63, which adds a constant equal to zero or to −Log_(—)1.6,Log_YL, Log_B*Y/Y. This 12-bit data stream is passed to anti-log table66, the output of which is passed through multiplier 67, which applies afactor of −1, +1,−0.5,+0.5 to make a data stream of the form(−YL*256/1.6), (R*256/1.6), (−YL*256/2), B*256/2. Add/pass block 68receives this data stream and produces outputs (−YL*256/1.6),CR=128+(R*256/1.6−YL*256/1.6), (−YL*256/2), CB=128+(B*256/2)−(YL*256/2).Line 69 then provides the desired chrominance output.

Although the invention is illustrated herein with reference to linearCCD sensors, the invention may be of benefit in any scanning apparatussubject to the same problems and tradeoffs as those using CCD sensors sothat limitation to CCD-based scanning apparatus is not intended.

The above descriptions and drawings disclose illustrative embodiments ofthe invention. Given the benefit of this disclosure, those skilled inthe art will appreciate that various modifications, alternateconstructions, and equivalents may also be employed to achieve theadvantages of the invention. Therefore, the invention is not to belimited to the above description and illustrations, but is defined bythe appended claims.

What is claimed is:
 1. A method of providing color values representativeof a scanned target object, comprising the steps of: generating a colorpixel array representative of at least a portion of said scanned object;generating a luminance pixel array representative of said portion ofsaid scanned object, wherein at lease one luminance pixel of saidluminance pixel array covers each color pixel of said color pixel array;sensing a luminance value of each luminance pixel of said luminancepixel array; providing a measured luminance value for each said colorpixel as a function of the sensed luminance value of the at least oneluminance pixel covering said color pixel; sensing three primary colorvalues for each said color pixel; calculating a derived luminance valuefor each said color pixel as a function of the three sensed primarycolor values thereof; determining a luminance correction factor for eachsaid color pixel as a function of the derived luminance value thereofand the measured luminance value thereof; and applying said luminancecorrection factor to a desired linear combination of the sensed primarycolor values of each said color pixel to determine luminance correctedcolor values thereof.
 2. The method of claim 1 wherein a plurality ofluminance pixels of said luminance pixel array covers each color pixelof said color pixel array whereby said luminance pixel array provides ahigher resolution than said color pixel array.
 3. The method of claim 1wherein said luminance pixel array is generated at a higher speed thansaid color pixel array.
 4. The method of claim 1 wherein said luminancecorrection factor comprises the ratio of said derived luminance value tosaid measured luminance value at least for all color pixels withsufficient luminance content.
 5. The method of claim 4 wherein thedetermination of said luminance correction factor includes the steps of:determining a logarithm of said derived luminance value; determining alogarithm of said measured luminance value; and subtracting saidlogarithm of said measured luminance value from said logarithm of saidderived luminance value to determine a logarithm of said luminancecorrection factor.
 6. The method of claim 5 the application of saidluminance correction factor to the sensed primary color values includesthe steps of: determining a logarithm of a sensed primary color value;and adding said logarithm of said luminance correction factor to saidlogarithm of said sensed primary color value.
 7. The method of claim 4wherein said luminance correction factor is applied only to red and blueprimary color values.
 8. Apparatus for use in color scanning forcorrecting the luminance of a target object, comprising: a color CCDsensor for generating a color pixel array representative of at least aportion of said scanned object and for sensing three primary colorvalues for each color pixel comprising said portion; a luminance CCDsensor for generating a luminance pixel array representative of saidportion and for sensing a luminance value for each luminance pixelcomprising said portion, wherein at least one luminance pixel of saidluminance pixel array covers each color pixel of said color pixel array;means for providing a measured luminance value for each said color pixelas a function of the sensed luminance value of the at least oneluminance pixel covering said color pixel; means for calculating aderived luminance value for each said color pixel as a function of thethree sensed primary color values thereof; means for determining aluminance correction factor for each said color pixel as a function ofthe derived luminance value thereof and the measured luminance valuethereof; and means for applying said luminance correction factor to adesired linear combination of the sensed primary color values of eachsaid color pixel to determine luminance corrected primary color valuesthereof.